Conventional PIR motion detectors have difficulty with "false" alarms resulting from pets moving through detection zones at close range to the detectors. The level of IR radiation emitted by pets is sufficient at such close range to cause an alarm. Usually care is taken to mask such zones or to disable motion detectors where pets are found. Clearly, such total or partial disabling of the motion detection alarm system is not desirable, however, it is considered better to lower the quality of detection to preserve an acceptable low level of false alarms. Alternatively, pet immunity has been provided in the signal processing of the PIR sensor response signal by ignoring low level responses generated by pets and/or requiring more movement before generating an alarm. Such measures risk failing to detect human intruder motion by effectively lowering sensitivity.
Another approach to dealing with pet immunity in motion detectors has been special processing of detector signals, particularly in the case of dual technology detectors, which may be used to distinguish between pets and human intruders, as is disclosed in U.S. Pat. No. 5,473,311 to Hoseit. Such detectors are more complicated and more costly, both at the sensor level and the signal analysis level.
Dual channel PIR motion detectors are known in the art, as for example in U.S. Pat. No. 4,614,938 to Weitman and U.S. Pat. No. 4,963,749 to McMaster. It is known to use a single quad PIR sensor having four IR sensitive elements as well as two PIR sensor devices each having a pair of IR sensitive elements. The advantage of two channels over one is simply greater reliability of sensor output signal. An alarm signal is thus only generated when both channels indicate motion. Preventing false alarms and ensuring detection is of great importance to PIR motion detectors used in the security industry.
In U.S. Pat. No. 4,697,081 to Baker, a quad element sensor is disclosed in which interdigitated IR sensitive elements are provided. By this arrangement, both IR elements respond to infrared radiation collected by the lens, and the risk of false triggering is reduced. In U.S. Pat. No. 5,045,702 to Mulleer, a single channel detector is disclosed in which the sensor element configurations include a diamond pattern with opposed pairs of IR sensitive elements of opposite polarization connected in series. Such motion detectors typically employ a single lens to direct infrared radiation onto the single quad or multi-element sensor.
It is also known in the art to provide dual lens and dual sensor motion detectors. Such systems conventionally have a single housing with two lenses mounted one above the other. Each sensor receives radiation from one corresponding lens. The optical arrangement is such that infrared radiation from a person entering a detection zone will not be simultaneously received by both sensors, but rather sequentially. The response from the sensors is thus separated in time, and has a same polarity since the sensor IR sensitive elements of the two sensors are aligned parallel with like polarity. Such a dual channel motion detector can generate an alarm accurately when the response in both channels is similar and separated in time by the expected amount.